Although the fine structure of malarial parasites has been studied by many investigators, there remain many unanswered questions. Re-examination of the morphology of the parasites by the high-voltage electron microscope together with stereoscopic analysis has been rewarding for understanding of various subcellular organelles. During the past year, we studied by electron microscopy the effect of immune serum on merozoites of P. knowlesi. When the extracellular merozoites were incubated in immune serum, the surface coat became very thickened, while normal serum showed little effect on the merozoites. For the coming year, we will study whether the membrane coating produced by immune serum enhances or decreases phagocytosis of the merozoites by macrophages. Also, we will study the effect of immune serum on the sporozoites and gametes of malarial parasites by conventional and high-voltage electron microscopy. BIBLIOGRAHIC REFERENCES: Aikawa, M., Miller, L.H. and Rabbege, J., Caveola-vesicle complexes in the plasmalemma of erythrocytes infected by Plasmodium vivax and Plasmodium cynomolgi: Unique structures related to Schuffner's dots. AM. J. Path. 79: 285-300, 1975. Sterling, C.R., Seed, T.M., Aikawa, M. and Rabbege, J., Erythrocyte membrane alterations induced by Plasmodium simium infection in Saimira sciureus: Relations to Schuffner's dots. J. Parasitol. 61: 177-188, 1975.